A back-side illuminated image sensor is devised in which a light-receiving surface of photodiodes is provided on the back surface of a semiconductor substrate, contrary to a front-side illuminated image sensor. In the back-side illuminated image sensor, sensitivity higher than that of the front-side illuminated image sensor can be obtained because wiring and many films need not be formed on the light-receiving surface. In the back-side illuminated image sensor, to efficiently collect light entering the back surface into photodiodes, thinning of the semiconductor substrate is required. For example, when visible light enters, the thickness of the semiconductor substrate is required to be thinner than 20 μm so as to prevent electrical charges generated on the light-receiving surface from diffusing and causing degradation of resolution before the electrical charges are collected into the photodiodes.
For example, such a semiconductor device is formed by the method described below. A semiconductor substrate on which photodiodes and integrated circuits are formed on its surface is prepared. A support substrate having substantially the same diameter as that of the semiconductor substrate is bonded to the semiconductor substrate. The support substrate is thinned from the back surface of the semiconductor substrate to a position near the photodiodes, and functions as a reinforcing body when the light-receiving surface is formed. Next, an antireflection film, a color filter, and microlenses for collecting light are provided on the light-receiving surface, and thereby a so-called back-side illuminated image sensor is formed in which energy rays such as light and electrons emitted from the back surface are received and collected into the photodiodes.
Further, electrode portions electrically connected to the integrated circuits of the semiconductor substrate are formed, and then the bonded body of the semiconductor substrate and the support substrate is cut and divided by a dicing blade. A divided chip is bonded to a ceramic package or the like, and the electrode portions of the chip are electrically connected with wiring formed on the ceramic package by wire bonding, and thereby a semiconductor device is formed.
In the semiconductor device described above, the semiconductor substrate is thinned partway from the back surface of the semiconductor substrate toward a layer on the front surface in which the photodiodes are formed by mechanical grinding, chemical mechanical polishing, and/or wet etching. To efficiently collect energy rays into the photodiodes, it is desired that the semiconductor substrate is thinned as much as possible.
However, when the semiconductor substrate is thinned, a wiring layer (formed of a metal material) is exposed on the outer circumference of the semiconductor substrate. If the wet etching is performed in that state, there are problems that the metal of the wiring layer is eluded into an etching solution, so not only the life of the etching solution is reduced, but also etching residue is generated due to reduction of etching rate and the yield ratio is reduced. In addition, there are also problems that, if eluded metal ions attach to an Si layer which is the light-receiving surface, the metal ions easily diffuse into Si, so imaging characteristics are degraded and the yield ratio is reduced.